Sigma-delta modulators (noise shapers) are often used as input stages for a pulse width modulator (PWM) in many applications, such as audio play back systems. More particularly, digital audio data is often stored on a disk, for example on a CD ROM, from which the data is retrieved and processed before being reconverted back to an analog audio signal.
To enhance the quality of the reproduced audio signal, the data is often "over-sampled," i.e., data points which were not originally extracted from the analog audio signal are interpolated in interpose between the data points which are retrieved from the CD disc. Various oversampling techniques are employed in the context of audio play back systems, for example digital interpolation using a finite impulse response (FIR) filter, which may generate interpolated data on the order of four to eight times that of the original digital data.
Another method for oversampling digital data involves the use a delta-sigma modulator (noise shapers) configured to oversample the data, again on the order of four to eight times the number of data points input to the noise shaper per unit time period. Digital data are applied to noise shapers, for example in the form of N data words at a predetermined sampling frequency Fs, and output more coarsely quantized data at a higher sampling rate. For example, a typical noise shaper may convert 16-bit data input at 8 Fs to 4-bit data at 32 Fs.
The 4-bit data output from the noise shaper may then be conveniently applied to a PWM module to effect sound reproduction of the oversampled digital data. Presently known noise shapers are configured to extract on the order of three or four most significant bits (MSBs) from a data signal, and apply the remaining least significant bits (LSBs) to a digital filter. By low pass filtering the least significant bits and thereafter summing the filtered LSBs with new input data, a coarsely quantized representation of the original data may be extracted at a frequency which is an integer multiple of the frequency at which the original data is input to the noise shaper. The coarsely quantized data is thereafter employed to address a ROM, from which predetermined data signals are retrieved and applied to a 1-bit DAC for subsequent application to an audio transducer.
Presently known noise shaping modules are disadvantageous in several respects. For example, conventional adders employed in known noise shapers are not equipped to output a carry bit. Thus, the adder may overflow if the sum of the input data to the adder exceeds the output capacity of the adder, resulting in corruption of the signal data applied to the audio transducer.
Presently known dual DAC pulse width modulator (PWM) systems are disadvantageous in that they often employ two separate ROMs for the purpose of simultaneously producing two separate signal outputs which are subsequently fed to respective independent shift registers and respective 1-bit DACs, the outputs of which are thereafter summed to produce a composite output signal. The circuitry required to implement these functions, and the propagation delay introduced thereby, limit the performance of known PWM modules.
A noise shaper and associated PWM module are needed which overcome the limitations of the prior art.